What would you do in a situation where you have multiple objectives, but limited time? That's one good way to learn how to split forces effectively.
Here's a scenario: there are 500 mk4 ships on a planet, and you need to destroy the warp gate, and the AI eye (and there are 3 guard posts on the planet). You have mk2 ships, so there's no way you can take on the enemy in a head-on confrontation, so you've dropped a Mk3 EMP on the planet and now have 120 seconds to complete your objective.
For me, I will assign all my ship types to different control groups - fighters (1), bonus1 (2), frigates (3), bombers (4), bonus2 (5), etc., and send each control group after a different objective. Mixing the forces isn't usually the best idea, because the frigates will slow everything down, and there's not really much point in sending fighters with bombers to attack a forcefield. If I have starships, I'll distribute them as-needed in the groups, but as a general rule light starships, and flagships will go with the frigates (They're slower than fighters and bombers - they may go with one of the bonus ships if they match the speed better/the situation needs the bonus ship to have the muniboost), bomber starships go with the bombers, dreadnoughts and raid starships are their own control group, and riot starships will usually form their own control group and hang around near the bombers in case things take a bit too long. Zenith and Spire starships I don't usually get a chance to unlock. IIRC Spires are fast enough to go with either bombers or fighters, but the huge range difference makes grouping them not that great of an idea.
When it comes to defense (I always play with schizophrenic waves), these control groups are necessary for me - if you hold down your FRD button and right click on an enemy ship, your selected ships will hunt down that specific AI ship first, and then move on to others. I'll assign my fighters to FRD hunt the enemy bombers (if the enemy has melee ships, those are actually priority, and I'll probably assign my bonus ship to hunt down the bombers instead), frigates FRD hunt the enemy fighters, I usually keep my bombers out of the fight (expensive and time consuming to rebuild, with short range and get eaten very quickly by enemy fighters), and bonus ship(s) hunt enemy frigates if the fighters aren't hunting melee ships. It doesn't so much work to blob in defense because your ships are all suffering from each other's weaknesses - your blob is going to suffer from low speed (frigates), making the short ranges of your fighters and bombers much less easy to work around, which means you're going to take more losses to your fighters from enemy frigates before your fighters get in range (because your bombers can't get to them), which means more bombers get through, which means problems for your fixed defenses.