Apologies for my ignorance but I've been looking up on the wiki the Romulan eps and I'd like to start them but when I go to T'nae they're not available. I'm a cmdr so they should be available. Are eps other than the Klingon ones not always available or is there some other problem here? The wiki is not clear on this and does not imply that the missions are sometimes unavailable.

Your question is not actually nooby. I recall many asking me the same over and over. To replay missions, press u and click the episodes tab. You will notice on the left panel different tabs for each story arc. Select whichever you want to play, scroll down/up to the mission of choice and click replay.

A few facts you need to know:
-Some of these missions have unique items rewards, which are removed after you've claimed them the first time.
-All arcs have to be unlocked by either playing through all the episodes of the previous arc or simply skipping episodes. As far as I know (not 100% sure), skipping doesn't affect mission rewards.
-All arcs have introduction episodes which have to be completed to progress in the storyline.

So I have to finish the Klingon eps before the other ones open up? Thanks, although it seems odd that some of these missions will are available from lv17 when, if you have to finish the Klingon eps, then you're probably already going to be a vice admiral. I'm half way through and a cpt.

Eps are always available once you have reached them or done them the first time the only thing that isnt available is the rare rewards which is only available during the rerun

I also stated in my previous comment that missions can be skipped (you just click the skip button). You really don't have to play all the missions. You might be blocked from some if you skip too much and find yourself outranked.

Players level really fast in STO. Nothing anyone can do about it. I became vice admiral myself during the early romulan episodes.

So far as I know, the only truly "mandatory" missions are the beginning-of-arc missions (e.g. the one starting the Cardassian arc).

Also as a note, there are four special mission rewards (the Devidian Cane, the Shard of Possibilities, the Reman Bridge Officer, and the Breen Bridge Officer) that are only available during "mission replay events," which seem to happen approximately twice per year. We're at the very tail end of one now (something like 3-4 more days). If you have a high enough level you can skip over the missions necessary to get to these points.

Personally I would say the Shard can be VERY helpful, so get it if you can; none of them, however, are by any means game breakers.

And the good thing about these missions, is that you only have to play the last episodes of the feature series to get those nice rewards .

The Shard of Possibilities is a great way to say hello in a social zone. Target your friend, click your Shard, and your duplicates will follow them around and dance!

Oh, and, it's really handy in combat too