Can different name brand AN fittings be used together? Like Fragola tube nuts with Summit brand tubing sleeves etc.

Maybe, but I would not recomend it.

I've used Summit, Earl's, and Russel fittings together but I've never heard of Fragola.

The specification for AN fittings is 37 degrees and the tube nut and sleeves should be for a specific O.D. of the tubing your using so # 8 tubing should be the same spec. regardless of which makers parts your using. I am mixing Fragola and Summit AN fittings on a project this week I will let you know if I run into any problems with mixing them, I can't imagine them not working together I have mixed tubing sleeves and nuts from other makers, Earls, Russel and Summits in the past and not had a problem with them sealing and working together, there is a slight color differences in the finish on them like Russel versus Summits

I did a lot of re-plumbing on my car last winter, oil, fuel and brake lines. I had to use Summit, Jegs, Russell and Earl's to get everything I needed. They all fit together but I have seen a big difference in quality and even had to toss a few of the cheaper brand fittings.