Dissimiliar Batteries

Go for it. A few years ago I split the DC bus on a Tiffin Allegro Bus and ran 6 Lifeline AGM's and 4 Exide flooded cells off the same setup. We used it for extended boondocking and ran them that way for 4+ years, about 55,000 miles. When we traded the unit the batteries were as good as when we put them in.

In simple plain English, its the voltage that will force different capacity batteries to charge and discharge equally. So they better all be the same chemistry/voltage.

I have seen may non-truths about mixing bigger and smaller batteries, But this is fact. Tested and proven fact.

It will work just fine, no problems with one possible exception.

Some folks yammer how one battery will charge faster.. STATE of charge is reflected in voltage, the battery with the lower voltage will "hog" the charge, so after the initial charge/discharge they will be in perfect lock step till one fails,, Some folks yammer about how one will work harder (true in direct proportion to their sizes) so it will run down faster (false see above).

The danger: Recently someone posted about a battery eploading, it was one of three (or more) identical 12 volt batteries in parallel, Best guess is a cell or two shorted in the blown up one and the others dumped their charge into it way faster than it could digest.. So it got gas and exploded, This can happen EVEN WITH IDENTICAL BATTERIES, (And in fact did in the story posted), it is also rather rare.

I run 3 different size batteries in parallel, no noticeable degradation of performance, all lasted around 9 years.

If the older battery has some wear and tear then the new battery in parallel with it will soon be in the same shape. Then they should stay like that.

IMO confirm what shape the older battery is in. (charge fully, get a load test, do your own hydrometer check) If it has been neglected like so many RV batteries, it could be in worse shape than expected. If so, it might be better to get two new ones and look after them.

If it is in decent shape just pair it with the new one.

Solar is good but the WFCO will probably be chronically undercharging the batteries.
Size will have virtually no effect.

That would be the plan, in a few years.

The 29 is less then a year old, the 24 or maybe a 27 would be new.

No problem as long as they are in the same condition for wear and tear. Both new makes that easy to know.

Mixing dissimilar batteries has problems where they are of different types or different conditions but just being different in capacity is no problem.

The charging equipment choice has many options but you just need to make sure the batts are getting charged up with whatever you have, which means use an hydrometer every so often.

Probably not enough to worry about! I would be looking at pairing them up in a Year or two!