I'm also starting out with Tyranids and although I have yet to try it out one idea that I've been toying with is to kit out one hiveship comletely with torpedoes and attack craft, and the other entirely with bio-plasma. Not particularly sporting perhaps but the though of slipping within 15cm and having upwards of a dozen shield ignoring lances does inspire some manical cackling.
This is a typical hiveship configuration for me, it's fun to field and plenty workable.
Afterimagedan is correct - the all bio-plasma hiveship does not work as well as it used to with refits, as your hiveships will now have absolutely minimal maneuverability, and be relatively fragile upon approach. However, it actually can work quite well, especially against any opposing fleet that also contains battleships. As long as you deploy correctly, enemy battleships do not have the maneuverability to avoid your bio-plasma ship, and and they have no choice but to brace or disengage when the two ships close - no ship can withstand that firepower, and going AAF past it is not a viable option when that means slamming into a cloud of escort drones. Against fleets without battleships it also can work well as a linebreaking ship and a fire magnet, no one wants to close with something that nasty, and since it pretty much just charges in like an angry bear, it draws plenty of fire away from your other ships.
I would suggest putting one set of port/starboard launch bays on your all plasma hiveship though. The two points of bio-plasma strength are not that significant a loss of firepower, but being forced to move half speed straight ahead can be devastating for a ship of this type, and having launch bays protects against this. Having the two fighters for defense can also be quite valuable.
The all-ordnance hiveship is nice because it can just hang back and provide support. There are a few things to be aware of, however. An all-ordnance hiveship is rather more vulnerable than it seems at first. If you brace with it, the next turn will be a complete waste, but if you don't and it gets crippled the ship will be
permanently useless. So if it comes under fire you'll have some tough choices. This also makes it
very vulnerable to nova cannons.
Also, if you're fielding an otherwise aggressive fleet (which you will be, since you're using an all bio-plasma hiveship too) you can't hang back too far with your ordnance hive, otherwise you will be wasting its synaptic control capabilities. Furthermore, if you're going to do all ordnance you really should have at least one or two other cruisers in the fleet with torpedoes. If you don't, your hiveship's torpedo salvo will be the only one in your whole fleet, and all too easy for you opponent to counter. I like to pair my all-ordnance hives with a twin torpedo cruiser as an escort of sorts. A torpedo salvo is ideal on Tyranid cruisers anyway, since it allows them to be relatively autonomous, so if you're making attack cruisers (or at least, the closest we can come to attack cruisers now
) try them with a torpedo salvo.
I the case of refits I do not really see the point of using them, I think the best tyranids ships are scorts because of their cost and weapons(close combat weapons are awesome 
), and you also need the permision of the game group wich means in some cases I cannot use them, I prefer to learn the hard way.
The point of using them is for cruisers. Imagine how IN players would feel if every cruiser option they had except for the Endeavor, Endurance, and Defiant was removed because Emperor-class battleships were too good. This is essentially what happened for Tyranid players with the refit ruling. The reason Tyranid cruisers aren't that good is because we're now stuck with short-ranged weapons on slow, fragile ships. Although six-shield, fourteen hit hiveships were pretty ridiculous, the lack of thought put into fixing that problem was equally ridiculous. But it is what it is, and I agree that unless you want ordnance, it's better to just use escorts now.
