I use a mixed bank that contains 8 Trojan 105 [220 AH 6 volt] and 4 SLA gel cells [115 AH 12 volt] in a 24 volt setup.

Probably not "ideal" but certainly works for me.

Bottom line, in my opinion, is that you use what you have / can afford. Best method is probably to buy all the same batteries at the same time. This is not always practical for most of us with limited financial resources.

In my case, I am careful not to "cook" the bank since I mixed types and capacities of batteries. When it was just the Trojans I did not worry about ramming a bit of over voltage into them because I could top those up on water, not so with the sealed units.

My biggest concern would be to be sure the 2 "sets" are at the same voltage so no current surges occur when you make the initial connection.

A huge draw or load could cause physical damage to the batteries.

An easy way to assure they are equal on voltage would be to connect the negative leads together then use something like an automotive tail light between the sets on the positive lead. This will limit the current to what the bulb uses. When the lamp stops glowing you know the batteries are at the same voltage and you can make your main connection safely.

Wear safety gear like glasses, gloves and old clothes and keep a jug of water on hand to flush any spills should you have a mishap. Or a friend ready with the hose to hose you and the area down should a spill arise.

Do not take battery safety lightly.

Good Luck.

Cheers.

TomW