50gph is really on the low side, so any upgrade would be good. You really have to keep in mind that if the max flow rate of the pump is 50 gph, you're going to get lesst than half of that, because a loop is very restrictive. If that's the case, you might have an actual flow rate of 20 gph, which is so low that you could switch tubing to 1/4" ID, without any ill effect! The block will perform a lot worse at those very low flow rates.
You can run two in series, or more, but you have to watch out for the total pressure that it puts on the pumps: most pump housings can handle plenty more pressure than they provide, but in series, you're just adding, and adding... which may cause the impeller housing to leak.
Idle and load water temps should be different.
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