Four inch Red ear slider..A few questions?

I got a red ear slider (RES) from my mother in law after she found it in her pool. I have him/her in a 55gal tank with every thing he/she needs to be healthy and happy. I have done TONS of Research on them. What I would like to know is mostly about the sex and age of my res. Its shell is about four inches long. It is still small but not tiny. Also the tail looks pretty long and thin (So I would think it is a boy) but its nails are not super long. They are kind of long,but not having another res of opposite sex to compare to makes it hard.Also I have been offered another tiny baby (about 1.5-2 inch) and would like to get one. Should I? I would let them meet in a neutral place and they would be going into my 150 gal when older..Plus will have an outside home in summer! I live in Arizona so great sun!! Yes I will make a nice shade place and hiding spots and it has a cover so no birds can get to them..Is there any way to sex babies if with the clutch for comparison. I know that no one will know for sure how old my res is sense you don't know the date of hatch,but I thought maybe you would be able to guess an approximate age! Also know that you can't say for sure what sex, but again asking for any advise. Another question is, how many night crawlers should I feed him/her at a time & a week/month...Like most she loves them but I don't want to spoil him,make him fat,or not to have the proper nutrition. I do feed live fish,pellets and veggies as well...Thanks for reading sorry so long..I like All info I can get!!