Old thread on "Image Circle" as an OTA's parameter
Indeed image circle size is not absolute. Acceptable image dimming (from fully illuminated, partially illuminated --> correctable by flat, to not illuminated),
various degrees of optical aberration/image degradation, etc. all play parts on an OTA designer/mfg designates that OTA model's image circle size.
As real life example, Canon EOS/EF mount lens has EF type and EF-S type. Latter is more suitable for (image sensor size smaller) APS-C sized DSLRs while former can be used for full-frame sized as well as APS-C sized DSLRs.
(As an analogy, Nikon has the FX type and DX type lenses.)
One could even argue that EF lens, when used in APS-C sized DSLR (a) will burn bigger hole in your pocket book,
(b) waste a lot of light on that APS-C DSLR, © probably a nice fit, since its smaller APS-C sized image sensor will pick up more of the
sweet-spot lights ( <-- one will notice near the photo's edge, the optical quality degrades more, so it's like using a more capable lens, then crop out the outer region of the image )
Back to OP's question 1 on which OTA can offer larger image circle, doublet or triplet OTA?
Ans: Hard to say, a high quality, pricey OTA may offer better optical quality in small image circle sized format and only willing to do so for reputation/good marketing purpose, while another brand/model may want to push the envelope a bit more to sell (and when customers complain, ask the users "just to crop the resulted image".)
Q2: FR, Barlow, C.C., etc.
Ans: These are OTA optics modifiers. They first need to match the original OTA's design parameters.
Still they will introduce all kind of effects... including F.R.'s vignetting (optical as well as mechanical), more aberrations, or improvements (e.g., coma correction if matching/proper C.C. is used)