In general, in 10's and 25's, you have about 2 or 5 healers.
Some of the healers are on the main tank or off-tank, others are on the raid.

If you're playing the main tank, it would be a lot easier on you, if your primary heal target was the main tank.
Have a different kind of healer (different buffs for the raid, less competition for healing gear) who specializes in AoE/raid heals.

If you want to use mod keys in macro's, you can combine these modifiers:
- [mod] specifies any or all mod keys are pressed.
- [nomod] specifies no mod keys are pressed.
- [mod:alt], [mod:ctrl], [mod:shift] specifies at least that key is pressed.
- [mod:alt,ctrl] specifies both mod keys are pressed.
- [mod:alt, nomod:shift] specifies one or more mod keys are pressed and one or more mod keys are not pressed.

You can really come up with a lot of combinations.
Probably more then you need for a single toon.

As an easier alternative, you could use your mouse buttons, such as thumb buttons on the mouse.
Or an addon like clique, to cast (heal) your current mouse-over target.