One simple way to test most rocks is to pour some vinegar on them, if it fizzes and bubbles it is not safe, if it doesnt then give it a good clean and it should be ok.
The only issue with sandstone is overtime, it will dissolve and crack leaving a mess in the tank. Slate is a good alternative to use along with texas holey rock or dry marine reef rock.
Ideally, if i can be avoided, you want to NOT cycle a tank with fish, it is extremely stressful for them and can affect their health long term, also you end up with fish that may not be compatible with what fish you want. Here
is an article on cycling which will give you an idea as to fishless cycling.
55g is a nice size of Cichlids, some more colorful easy to keep cichlids which would work in that size tank are African Cichlids.
Yellow Labs, Acei, Rusty, Red Zebra would all work in that tank and give your grand kids some colorful fish to look at. In that size tank, you could keep about 12-15 fish.
If convicts are what you want, then there are a number of tank mates you can keep in that size tank.
Jack dempseys(ONLY one as they can be aggressive to other fish)
Green Terrors (You can keep a pair in a 55g provided they are already mated)
I would only get a MATED pair of convicts, or several juveniles and let them pair off, once a pair is formed, REMOVE the others, convicts are aggressive to other convicts when paired and sexually mature.