First of all i would say, its not wise to buy a fish like Indo's or a Silver Aro unless you can completely cater for it, without a lot of compromise's such as risking small loach, only having a 60G and restricted acess to food stuffs.
BUT, to answer you questions:
You could keep an Indo in a 60G if bought small (few inches) for maybe 1 year to 2 years at a push, basically untill it reaches 8"-10" or so.
Indo's will grow more slowly than a silver and will top out at 12-14" given enough time maybe bigger, when the silver will reach 2ft quickly and given enough time, room and food get to 3ft.
Clown loach grow very slowly and and the largest dat you could keep with 1.5" clowns would be about 6".
The Indo would be harder to train to take pellets than the aro, they can be fussy feeders at firs, if bought small, but it depends largely on the fish. Training isnt too hard, you could always dig up some worms for free if all else fails (although this can be qite risky, due to possible chemicals etc in the soil) so maybe get them from a bait shop, or somewhere like worms direct.
There are a lot of things that dictate the colour of a dat, like stress level different environment (decor) water perameters etc but im not sure about silver dats and the depth in colour of markings as the fish matures so someone else will have to answer that one
I wouldnt reccomend them to a newbie to the whole hobby but if your just new to oddballs and have a reasonable ammount of experience with other fish then neither are vary hard to keep, providing you keep up to your water changes and provide a good diet.
Remember you will need a large ammount of filtration to cater for fish like this, a quality external canister like an FX5 would be good on a 60G although the type of canister is largely down to personal presferance, im an FX5 man myself lol
Oh and the 1" per Gallon rule doesnt apply with large fish like this, they need plenty of room.
GL
