Am i right that you want a turtle?
Well its important to know what exact specie you want...do you have latin name?
You have turtle who rather eat meat and other who go for the green
Normally you should be fine with giving them vegtable left overs...they don't get special shit in the wild either
Though you can look up in petshop for some turtle food and change between that and normal vegtables/fruit.
Or look for some vitamins you can add to your food.
Look for special vitamins for turtles it should be indicated on the botle/pack how much you have to use.
They will probably suggest more then is really needed but to much vitamins usaly don't hurt.
Like with humans...what you dont use go's out again
Other thing is that special for turtle crap will cost 3 times more so well you can as well make your own vitamin mixture once you know what you need
But it really think that you should just gave him varieted/allot of diferent kinds of food.
I'm guessing here but is chalk used for armor? as well if chicken lay eggs with to thin scale you can add fine crushed egg shells to their grains (you have to dry scales) out in oven or something so maybe you can do that as well
would be easy and cheap anyway
I think they would like bugs to....So maybe they even like those snails if your lucky
Most of them are not very mobile so small fench around cracks where it can escape should be fine...But keep an eye on your turtle first days
For the water well try to make it as deep as possible (for oxigen and so on) and keep it in shade
Greate a soft going in so it can get in and out easely and make him litle private island in midle so he can chill without to worry
Cats and dogs well it all depends on how much those are interested in the turtle and watch out some turtles can be quite agresive/strong so again be sure to know what type you take and how big it will become.
My cat and dog live well with other creatures living in and around my house.
BUT its very important that they see that other animal as part as the family itself...sounds strange maybe but normally they should see it as a person and so not attack your new friend/you...get it?
So maybe let them both get used to eachother before they actually can touch eachother
For the rest....look good at your new friend and learn what he want and need
For interaction...I dont know I even saw big bugs and ppl "comunicating"...I think most will depend on how creature normally lives (group/solo/...).
So I hope I could tell you something with this
Good luck anyway
but, that is the opportunity to learn 
