they all have their flaws but Kalter would be the best player to train. Jose has atrocious passing and handling which is horrible to look at. Big men need at least mediocre passing in my eyes, or at very least inept. Kalter alright inside skills, great height, just dont like that OD or JR. If a team plays a PF at the C spot and that PF has good JS or JR then he will give up a lot of points. Don't bother training them though, unless you really have or want to. Do passing training for him. Stone has good inside skills and decent passing but lacks all other guard skills which isn't good. With that said though for a inside team who wants 1 really dominant C he would be good for it. Good inside skills and passing. Your last player flowers is quite good at inside skills. Bad passing and OD. I would only keep him if you are planning to do passing this season.
With that said i'd say sell flowers or Jose. I hate double atrocious for a C and Jose would prob go for more than the others, besides Kalter. Flowers is your weakest trainee. You may want to sell both and buy another 18 yo trainee, thats up to you.
For training definately make Kalter your 1st trainee. Develope your team around him. With his potential he can have good inside skills. I would train passing first but that height is horrible to train passing with. Still do outside training starting with passing then do one on one for a little bit. After your happy with those guard skills do inside training. Kalter could possibily cap or salary may get to high for you if you just do inside skills. Its better to do outside skills first on big men then focus on inside skills. Good luck, and remember to manage GS, you have 2 guys on mediocre