quick plumbing question

[Spiny]

1: it will only be too small if your return pump is too strong.
2: You should install gate valves only where you need them. Personally I went with ball valves instead, less expensive and they do what I need them to. Very useful to throtle back your return pump.
3: Once again, $$$. If you can use them, by all means do so and you'll save yourself headaches down the road, but unions are nearly 10$ each, so it adds up pretty fast. If I were to do over, I'd use slip fittings everywhere except for a union right at the bulkhead to make for easy disconnect.