Ideally the older slightly used batteries would be charged separately and fully before connecting them in parallel series with the newer batteries. Hopefully the resting voltages when fully charged would be within 0.02v of each other. When the restiing fully charged voltages are more than 0.1v difference then one either really needs to get the lower batteries up through a long equalization charge, and if not possible, then should not parallel them with the Newer batteries, as the newer batteries will always be charging the older batteries, slightly.
New and old batteries in parallel are never ideal, how how unideal they are depends on how aged the older batteries are.
Considering no two batteries off the assembly line are exactly the same, the recommendation that old and new batteries HAVE to be same age make and condition is a little unrealistic, but it is not wrong.
Old batteries can certainly drag down new batteries when in parallel, and they can also make it hard to fine tune absorption voltage and duration as when the older batteries finally max out the SG, the newer batteries might have been there for an hour or more, and that hour they were using more water and having their positive plates migrate to the negative plates, unnecessarily.
Fine tuning absorption voltage and duration is the key to battery longevity, assuming the charging source can attain absorption voltage and hold it long enough.
The capacity is King mindset is not always right either. Too much battery for too little solar should not give the warm and fuzzies, especially when the bank is drawn below 80% SOC often
Keep in mind Trojan's 10 to 13% 'recommendation' which assumes a plug in charging source. This would be about 25 amps for a Single pair of T-105s when draws to the 50% range, which is about 450 watts of Solar.
Rolls Surrette make the worlds best Flooded lead acid batteries. In this extremely well written article, they recommend way more solar wattage to battery capacity than is commonly accepted by the Off grid community who use to say 5 to 13% was the acceptable range.
http://rollsbattery.com/uploads/pdfs/documents/user_manuals/Rolls_Battery_Manual.pdf
please read from page 26 ON regarding solar charging of batteries, and read the whole thing twice. If one can understand it all, then one would know everything possible about lead acid battery care.
A
Renewable Energy PV system should be designed to provide a charge current that is
capable of recharging the batteries quickly, efficiently and within the window of time
when the system is generating peak power (peak sun). The charge current should be
within 10-20% of the 20 Hr AH rate (C20) rate of the battery bank
This is Much much higher than the often stated and recommended 1 solar watt to 1Ah of battery capacity.
200 watts for 200AH of capacity is about a 5% rate.
Rolls Surrette recommendations are not to be brushed off lightly. Do so at the expense of battery life. Dialing in absorption voltage and duration on lightly cycled large capacity banks with too little solar wattage, will greatly offset having too little a ratio that can achieve a 10 to 20% rate.
When I lowered my battery capacity to better meet my Solars maximum ability, I came close to 10%, and my cycles per dollar ratio was significantly more agreeable than more feel good capacity cycled shallower.
So my point is, more capacity than needed might make a person feel better, but it might not save them a dollar, and could very well cost more money for those warm and fuzzies of having a lot of battery capacity.
I've gone from 345 total AH capacity, to just 90AH total, and I take 45 to 60 AH each night. But I employ high alternator currents or a 40 amp charger whenever my solar has no chance of replacing those 45 to 60AH + ~5% more.
Nothing to freak out about, having more capacity than needed or which is ideal for the charging sources rate, just be aware it is not ideal, and when setting up a new system, the typical 1 to 1 ratio, with 50% discharges, is way too little to keep the battery happy.