First of all, welcome.
You should try some games against human opponents because it is usually more fun.
A good player will use whatever units fill a hole in his strategy. It really depends on each individual's playstyle. Spamming just a couple of unit types cuts down on teching costs, which is an advantage, but leaves you with a less diverse force meaning that you will find it more difficult to react if the enemy suddenly changes unit composition. If you did happen to play a human, using the same few unit types would lead him to adapt to your strat and build counters and if you don't change strat he would beat you.
Tank hunters are not absolutely required but they are a good addition to your forces. They stop units like armoured cars from flanking your AT guns which would otherwise find it difficult to deal with these. By combining the two, you make them a very effective counter to both tanks and light vehicles no matter how the enemy uses them. Mines are also good for stopping your AT guns from getting flanked so it can be good to lay down a few of those if you want to skip the TH research.
