If the Romulan fleet is uncloaked they are a slightly more expensive Plasma race with good manoeuvring.
If the Romulan fleet is cloaked they are very tough but have sacrificed all tactical control of the battle and are playing turtle.
If half the fleet is cloaked and the other half is uncloaked then you have them outnumbered two to one as long as you stay clear of the cloaked ones.
Tactics very much defend on what the Romulan is doing. An entire fleet under cloak that is spread out allows you to flank them and pick on a few while the others are out of range. Maybe an APE on turn one to run as many of your ships as possible to one end of an extended Romulan formation. As has been mentioned pick on the Olde Schoole Rom ships first as they are very squishy under the shields. If facing a modern fleet go for the escorts first, the cloak will reduce your firepower a lot so go for the ships that will be most hurt by a few hits.
Don’t focus too much on shield penetrating hits, between natural 6s and the cloak it will take ages to do anything just with natural 6s. Look at the possible targets, a ship with 16 shields and 6/2 compared to a ship with 12 shields and 10/4 is harder to take down unless you get those natural 6s. With normal damage from Phasers and Photons it is easier to criple the 12 10/4 since 18 damage will do it where as waiting for natural 6s may take longer on the 16 6/2 ship. Remember that a cloak rolls agains the number of Hits of damage not against the number of weaposn that hit. Three phasers inside of multihit range do 6 Hits, that is an average of 1 Damage after the cloak. Many fed ships have 6 Phaser ones in arc, four Photons with two hitting add another 8 or 1.3 after the cloak (overloads for more fun). Four damage isn't a lot but if two of your ships do that to one of his smaller ships and then do the same on the next turn that is a shield down.
If he stays under cloak pick on one end of his formation till he bunches up. If you can take out a few of his small ships you can Init sink him and at that point you can concentrate on one of his at a time after you have made him move his fleet.
Once you are in a position to Init sink (or before if you have agile escort sized ships) get some ships round behind him and start to play with overloaded Photons. Watch for any ships declaring an Uncloak SA and keep out of range of them but otherwise you can cruise into range and dump full overloads on the cloaked ships. With overloads you are looking for shield penetrations but also just smashing down the shields of the smaller ships.
A Photon overloaded is going to do 8 Hits, fire two or three from a small ship plus a few Phasers and even with a 2+ cloak save you can do some damage. Several ships firing at a single target and you can take down the shields of and wreck an escort sized ship in several turns. If the Rom ignores you just kill his small stuff one by one, if he uncloaks part of his fleet to attack your small ships the rest of your fleet has a target.
Remember that it takes the firepower of several Plasma ships to kill an equal sized ship in most cases. Two of your escorts will need the firepower of several Romulan ships to chase off which means unless he uncloaks his entire fleet you have just gained a few targets to hammer.
If he stays bunched up in a tight formation and wanders round under cloak get a book to read. Fire Phasers and photons from maximum range, pick one target and hit it with everything every turn then run away to keep the range open. Bring a blanket and cushion so you can sleep through the turns as this is going to be boring………
Re the advice above about 12”, the magic range is 15” if you can do it. Cloak jump is 6”, plasma short range is 8”, 14” is a cloaked ships kill zone. Remember that a ship that is uncloaking can be placed anywhere within a 6” radius of its cloaked model and can make a 45degree turn (90 degrees if agile). This means agile Rom ships can uncloak and hit a ship 14” away to the side or front. Another range to remember is 7”, if you can get behind a Romulan ship even if he jumps directly towards you he still cannot make more than a single turn, many Romulan Plasmas are F arc so you are safe from everything but his side arc Phasers.
Uncloaking is an SA so you know which ships declare it, make sure you are more than 14” from them.
Probably the most scary Romulan’s you meet are the ones who do NOT start cloaked, they go APE, rush your flanks, dump massive plasma salvos on a few targets and THEN cloak to reload. Some agile Romulan ships with decent Phasers need never cloak unless they are clearly about to be surrounded and murdered. The cloak is an excellent defence but as they say “The best Defence is a good Offence”.
The Romulan that gets in your face, smacks you in the teeth with plasma then dances off to reload and only cloaks the ships that you are picking on is the Romulan you need to worry about. Turtles are tough but as long as you stay far enough away they are unable to hurt you much and just a matter of time to wear them down