The RFG will create some back pressure as any educator style nozzle will, but depending on how much flow you push through the RFG it can be relatively minim. At your "Optimal Suggested flow" for the 1/2in RFG you can expect around 10 to 15 throughput loss from the return line, but you gain that back in in terms of volume due to the eductors.
It's important to note that all of our suggested flow ratings are "To the Nozzle", meaning, we measure the flow BEFORE we add the nozzle for the rating.
Also - given your pump size, you should have plenty of flow to drive two 1/2in RFGs. Here's a quick flow Demo video of he 1/2in RFG at approx 500-600 GPH to the nozzle
Next, in terms of size, the whole assembly (19mm Slip-fit-drop adapter with the 1/2in RFG Nozzle) is slightly longer than the Spin Stream, but is basically the same length as the stock adapter and flare nozzle that originally came with you tank.
And finally, if you do have very small fish, or even fish or inhabitants that are not to strong swimmers, it is possibly for them to get stuck in or sucked into the eductors of they get too close. However, this is pretty rare but if it;s a concerns there are ways to help mitigate the risk, such as the foam mod we outline in
this video with the 3/4in RFG
hope that answers all your questions, - if you have any additional questions, we'll be happy to help.