Yes of course genitalia has to match the base model, but all models you find will be based on V7 or V8 aka G3-G8
so whatever young model you will find you can use adult genitalia. V8 genitalia is also usable for V7-G3
Yes, you can apply growing up to any model, but there is a catch that those morphs will typically mess up your long hair since they alter head/shoulders proportion.
so if you use long hair it may be preferable not to fit the hair but just parent it to the head or use short hair. Some long hair actually works fine.
You can find AngelWings models almost anywhere just google them. his official shop is on renderosity.com to check wahst is available.
Generally, I would recommend to check 3d-load.net site and quickly fill all your model needs. it has everything you may use. so do not even bother downloading everything in a row, just chose what you can actually use.
You may have a problem with boys as it is not that easy to find matching genitalia textures but there is a solution as well if you need it about which we will talk later if necessary.