Considering that they have similar coat colours, are guards, and have similar first names. Could they be related?

I say yes

6725290
So far as my headcanon goes, yes.

So far as the likelihood within the series goes...it's not impossible that Flash Magnus is Flash Sentry's ancestor. We know that Mage Meadowbrook has descendants in the modern day, after all.

6725290
I think so

6725290
I say it's more than likely. Maybe certain pegasi name their children after their ancestors as a sign of good fortune?

6725312
Don't forget Stygian, he has a descendant named Shadowlock in the IDW comics.

6725290 I've been thinking the same thing. Having an ancestor like Flash Magnus would explain how Flash Sentry could have such a high ranking position within the royal guard when he seems to be roughly the same age as the mane six.

6725290
I'd be willing to believe it honestly.

If Pegasi name their children based on the personal name last and family name first (say, Rainbow is the clan name and Dash is her personal name) then Flash Sentry may be Sentry of the Flash Clan and Flash Magnus could certainly be an ancestor.

6725290
That would be an awesome idea in the show!

6725336
Yep, him too.