Your chicks might appear to be doing okay at 75 degrees but they are probably being stressed at this low temperature and would be better off with some sort of heat source.
The general rule for brooding chicks is 95 degrees the first week, then 90 the second week, etc. Keep reducing the temp by about 5 degrees per week, and by the fifth or sixth week they no longer need a heat source.
It's not too late to add a heat lamp, and you won't need a very hot one if the temperature of the surrounding air is already 75 degrees as you mentioned. If possible use a ceramic lamp socket with an aluminum reflector hood (check your feed store or hardware store). Reflector-type brooder lamp bulbs typically come in 250 and 125 watt sizes. In your case you definitely don't need 250 watts of heat, so go with the lesser wattage bulb. If you can't find the 125 watt brooder bulbs, just use a regular soft-white light bulb (60, 75 or 100 watts only, no higher). Suspend the lamp at least 14 inches from the floor in the center of the brooding area and raise the lamp as needed to maintain the desired temperature (measure the temperature close to the floor under the lamp). ALWAYS make sure that the chicks have enough room to get away from lamp in case they get too warm.
You actually don't even need to use a thermometer - the best way to tell if your chicks are at the right temperature is to look at how they are dispersed - if they are huddled together in a group, they are too cool (or the brooder is drafty). If the chicks are all situated as far as they can get from the heat soure, or if they're panting, then it's too warm. If the chicks are evenly scattered around the brooder (a few napping under the lamp, others napping elsewhere, others at the feeder or fountain, or exploring, etc). then your brooder temp is about right
As for grit, you will need it as soon as you start feeding any food that is not pre-ground (mash or crumbles are generally considered pre-ground foods). So if you add greens, cracked grains, etc. you should add some insoluble grit to the diet. Ground granite comes in several degress of coarseness and is available at feed stores. The finest grind, called chick grit, is occasionally hard to find. You might even have to special order it, but a little will go a long way, so 5 or 10 pounds would be plenty. Sprinkle it over the feed like you were salting your food; do this a couple of timees a day and that should be enough. They can be switched to the coarser "grower" grit when they are old enough to leave the brooder.
Hope this helps.
Edited to add: Forgot to address your question about parakeet grit . . . I think it would probably work - chick grit is about the size of VERY coarse sand, so if your parakeet grit looks like that then it should be OK. Just don't confuse soluble grit (like ground oyster shell, used as a calcium source for egg-layers) with insoluble grit (needed by all birds for the crop to be able to grind food).
