1. If you combine an RGB or LRGB image of differently looking images then the colors will be falsified/crazy. If you use Newtonian spike version as L where as refractor for RGB then the spikes will have no color data and will look weird, especially when RGB will have black there. And when the size will differ you will have color fringes around stars and it will be super hard to scale that in a way that non-star objects match. In short it's processing nightmare. Refractor + Refractors are more likely, although they should have similar characteristics so that so you don't have to fight with star sizes. Or just shoot L in bin1 while RGB in bin2 from one scope.
2. Having multiple scopes is not equal to using every on the same target for the same image. You blindly follow image tooltips or somebody hardware list.
3. Wasting 3 years = hardware isn't the problem, rather that inexperienced user. Many years ago I just bought a good starting kit and got to planetary imaging night one, took some time to get used to the whole process but I did not had to buy and buy stuff, then I bought Meade DSI III Pro as super entry level DS camera, hooked my planetary camera as guider and had M27 first night - it didn't looked like from a $10 000 setup but it was clear and sharp. You don't need extremely complex hardware to do DS imaging - and in other threat you literally asked for no replies other than those agreeing with your shopping spree. And you moved from asking in astrophotography forums to low traffic hardware forums where is less likely to get proper responses.
Don't buy, just ask for help with problems, as 99% of the time you don't have to buy to solve them.
1. RGB or OSC will be done with the Newt, Newtonian is perfect for colors, and i will get spikes ion color, Lum and Ha will be with the refr [or any second scope, if you know how can i match another 8" F/5 Newt spikes with first one please go ahead and i will buy that second 8" F/5 in no time for L and Ha or half filters of LRGB].
2. I follow somebody list is nice and i like it, he or they told me they came long way and they made it and it can be done, actually they told me not listen to others who said it is not possible or not practical, so i listen to those who did it, and not necessary i have same gear list as theirs, but they used multiple setup just fine, i can give it a try first before i continue to more setup gear, 90 APO is my first ever triplet refr, it will be my main refr over my ST80, and i will buy FRA400 because i like it with and without a reducer, if i managed to match both scopes together in results successfully then i can do the same for more, if i failed on that then very simple i won't do that for 6" or 8" scopes, it will be a waste, but i didn't try/test yet, why giving up before i start just because you said so?!!!
3. I looked at your link, you are doing fine, but i didn't see something that telling me about all your comments to me are valid or well taken, i feel like you only giving advises based on what you read and searched and maybe your own experiences, so if you failed in something you tell others also not do it, i have to admit that the three years was my fault, but gear also cooperated for that fault/fail results, so i didn't want to depend in my skills and let the gear do the job, i improve and learn my skills by the time, but i want my hardware to be as good, and when i saw what people can do with processing even a so so not good data i just ignored my skills and only thought about to buy certain gear according to my budget, i have no regret of buying 180 Mak, no regret for that 8" and 6" Newt, they were so **** cheap anyway and bought them right time, now waiting this 90 triplet to be added to the collection and see, someone had better gear than me and spending also years and didn't get that results which those gear should be for, i asked him, he said he doesn't care to have high quality APOD like results, he bought those and keep imaging, and he told me that i am doing better than him and i should keep buying gear to what i think, i didn't spend fortune yet anyway, 2 cheap scopes[$200-300] and another 2 at reasonable prices [$1000-1300] isn't a big deal, heck even my ST80 i am happy with.
Problems are there with high end gear and low end gear, i didn't ask about problems, i already asked about problems while ago another places, got answers, so i moved now from problems solved to buy more stuff for fun, it is not only about buying more to solve more issues, you got that by yourself not from me, sorry if that what you thought about me or from my questions, but i myself have to tell you that i am buying more only because i like to buy more, and others couldn't solve all problems anyway, sometimes some problems is my own and i have to do my way, again, dual imaging isn't a crime or bad, i am sure you are so scared about doing it because of issues, that is limiting, using one scope even so fast one never fixed my time problem as well, this problem time needs different approach, if you don't like it then let/leave others to do and let them face their business, i did use my Canon 135mm lens at F2 with only LRGB and Ha filter all at F2, you know what, i spent 3 nights collecting data and i am still not happy and i was so tired, and many just telling me go with RASA because it is fast, if a lens at F2 didn't make it for me about time, why a scope at F2 will do then, but when i tested ONLY ONCE my ST80+0.8x reducer next to my Canon 300mm at f2.8, i got bad data, but guess what, i go the data in 2 hours that i have to spend like 4-5 hours to do, that is alone what made me to keep this multiple imaging system plan going, don't ask me why bad data, it wasn't a test for having good data, it was a test about the time of collecting data, i collected Ha and OIII and SII data in 2 hours, when i was using Canon 300mm at f2.8 alone i spent like 2-3 nights to collect SHO data, and it wasn't any better anyway over two scopes/cameras in one night.
Back to my topic, if my soon coming system working as dual imaging system, then what i can get for 8" Newt for dual imaging system as a scope? Easy question really.