The SSI diffusers you have linked are spec'd to flow 0-12 CFM. I would worry about running them at the max of 12 CFM, but they should easily handle 2 to 5 CFM. Keep in mind that I am not familiar with the quality of the SSI diffusers, but just by looking at the specs...they can take some air flow. Same applies with the quality of the SC50 piston pump...I'm just not familiar with either brand.
I believe you are right in looking for a better pump to run two or more diffusers. The EcoPlus 7 should be an affordable entry level pump for a single diffuser at 8 foot depths. The SC50 piston pump would operate your diffusers in a much better fashion. The max flow of the SC50 would work very well with two diffusers at about 2 CFM per diffuser, placed in 10 foot of water, a bit more CFM if in 8 foot, but still within the diffuser specs. It's a good choice for what you are doing.
You might save some money by looking at the rotary vane pumps. The Gast brand gets a lot of kudos here at PB and is what I use in my pond. The Gast 0523 model 1/4 Hp pump would be the right size to check into. It's been a while since I purchased mine, so the price has surely gone up. It won't pump air to the depths that the piston pump would , but at 10 foot of depth, it would produce similar air flow as the SC50 and is a very reliable pump that can be rebuilt rather easily given it just wore out.
Another option would be to look in to going the cheaper route and getting two of the EcoPlus 7's (or a larger model) or research the equivalent in the HiBlow brand. There are plenty of people using the HiBlow's to areate their pond. Both are diaphragm pumps that can be rebuilt easily, but diaphragm pump curves typically indicate that they are not the best suited for pumping air down to 8 to 10 foot depths. Meaning, they lose air flow in deeper water and would be operating outside the manufactures recommended pressure spec. A generic statement, but pretty typical.
My advice would be to stick with the piston pump or the rotary vane. They do produce more noise, but are more reliable. Some say that they consume more electric than the diaphragm pumps, but being a mechanical engineer...electrical concepts are more challenging...lol
