First off let me state that I'm not real familiar with this particular piece of equipment. That said, restricting of pumps, for whatever reason, is fairly common. As long as it's done on the output side of things, most all pumps are fine with it. They are magnetically driven, it's not like there are gears or anything to stress out. Short answer, I'm sure that will be fine without you modifying it. Plus you never know, after the tank grows in a bit you may wish you had all that flow back. I permanently modify equipment only as a last resort.
I don't see any issue adding your plants immediately to the new setup. Matter of fact, I recommend it. An argument could be made for waiting on the tank to cycle a bit before dropping in the Betta, however, me personally...I'd toss him him in as soon as you condition the water. Bettas live in terrible conditions, they are tough and breath air half the time. I would keep and eye on things and change out some of the water every other day for the first week or two though.