can I make a full game for mobile then add multiplayer last? just want to make sure I don’t waste money.
You can make your game for singleplayer and then after finishing it you can turn it into a multiplayer but I dont prefer that because it will cost you a lot of time converting to a multiplayer and you have to change some mechanisms and techniques you have used for the singleplayer such as child-parent stuff and root objects, changing holders of some scripts, techniques inside the script. You can avoid all that complex by just jumping strait into the multiplayer from the start. Changing a single player game to multiplayer will cost about a whole week of work without breaks (I experienced that) I hope that helped you 
Hamza Odeh
multiplayer game is best made with programming from the start. If u have little to no experience coding one, I suggest you try to make a few multiplayer games simple ones to start with. I made a car game (got inspired by need for speed) in multiplayer for lan and found that making multiplayer games vs singleplayer has a steeper learning curve, I decided to make a basic multiplayer game with what little unity3d knowledge I had. I used HLAPI for it and went through a lot of coding and debugging before ending up with a game. Thoroughly enjoyed playing the game afterwards.
Wish you all the best