No technical reason against different capacity batteries in parallel.
I have a document from Exide that gives acceptable chemistries that will work OK in parallel. eg AGM in parallel with standard flooded cell lead acid work well enough, but GEL in parallel with just about anything else is a no-no.
Other factors mentioned, like large differences in age age and slightly different chemistries just mean that the result will not be optimum rather than being a complete disaster - but unless you have bottomless wallet, all new batteries, same age and same brand, will not be the optimum solution for you and like most of the rest of us, you will likely make the best of what you have.
Identical length leads are another "nice to have" feature that is somehow evolved into a "must have" feature and unless you are charging and discharging at high rates, isn't really a practical consideration.
