The easiest and safest choice of shrimp with the dlue dream would be Tangerine Tiger (assumig you can get hold of some), they like similar water and are as easy to keep as neocaridina but the two don't interbreed and are usually fairly reasonable in price! The contrasting colour will also make it interesting to look at. I've not tried tiger shrimp yet myself but with all the disruption here because of the virus its not a very good time to get shrimps through the post so I will hold off for now!
As you say/know guppies are very curious fish and will get baby shrimps so you will want a densely planted tank with lots of hiding places if any of the baby shrimp are to survive. As you also say, it is a shame the guppies can't go in with the Betta as that would solve the shrimp problem, but guppies and Betta don't go well together - there is always some problem stopping you from getting everything as you would like in this hobby?
If you can resist the temptation to get the guppies for a while it may be better so the tank can settle down and your shrimp can multiply and you get enough of a good size shrimp to give them a chance? There are always many problems that can arise with new tanks and new livestock so it would also make sense from that point not to do/add too much early on as well? If you plan to get the shrimp shipped then they usually send fairly small and young shrimps that way so bear that in mind! Also, of course, the shrimp will naturally hide a lot if they feel unsafe with too many preditors in the aquarium?
Now the aquariums. I can't find a fluval spec, but the fluval flex I know and had a similar setup years ago with the equipment hidden in the back. It is very clean and tidy looking, however there are a few problems that can arise with small shrimp. There are slots top and bottom where the water is drawn in to the back, but these slots aren't shrimp safe, shrimp can get through, or even get drawn through when young. You will need to get some sponge to put behind these slots to make it shrimp safe, but not too fine as that will reduce the amount of water circulating through the back section. This is only important/needed with small shrimps like neocaridina, the amanos are way to big to need to do this? This extra sponge needs to be removable for squeezing during regular maintenance as it will otherwise get clogged/blocked. You will need to check the water level in the back regularly as it can drop dramarically, which could be a problem if the heater is in the back!
I don't know the setup of the 'spec' aquarium as a search only showed small 10L tanks but they had the same type of filtration system as the flex.
The flow for the betta may be able to be adjusted on the pump, mine was (though it didn't have a betta in it, just shrimps). You should be able to reduce the water disturbance also by pointing the outlets upwards and to the sides and that may be enough (hopefully). If these 2 aren't sufficient then you can cut a piece of sponge (again, not fine sponge) to cover the outlet (you can remove the standard flat end piece to make that easier). My betta tank has a spray bar and I have a sponge sleeve on that. Again though, keep a close eye on the water level in the back section though, mine dropped to half way at times (as the sponge gets clogged) even though the main tank level never dropped at all!
Simon