4 Instant Spells

Hey I was going to create 4 instant cast spells, I was wondering which spells would be best to base it off of. I'm not the best with creating spells, but you cannot base 2 spells on 1 spell if its on the same unit correct? Meaning like... Spell 1 and Spell 2 are based off of Roar. If I were to cast one, the other would mess up condition checks right?

You are correct.

Instant cast spells:

Roar
Fan of Knives
Howl of Terror
Battle Roar
Summon (Water elemental, bear, spirit wolves, etc.)
Channel

Your best bet is probably channel, because you can configure what kind of cast you want, it has an instant option. You'll need to change the base order ID's of each spell, otherwise there would be conflicts, because the unit would cast all of the spells that have that base order.

also, the berserk spell is the only good dummy INSTANT spell; it has no animation/casting/w/e, so it takes almost 0 time for the "player starts the effect of an ability event" to fire when its casted. it DOES take some time to cast tho, though this may be VERY VERY short, so it can be stopped still by "unit issuedorder/unit beings casting an ability ---- issue stop order".

windwalk is instant too, but it makes the unit invis for a while, even if u set duration to 0.01. this could mess up the orders of units targeting the unit that casts the spell.

immolation, defend and manashield are also instant spells but you can only detect when the orders to toggle them are issued, and when the "on" part is casted; you cant detect when they "off" part is casted. (you can detect "unit is issued "immolationon"/"immolationoff"" order, and you can detect "unit starts the effect of turn on immolation", but you cant detect "unti starts the effect of an ability" + "ability being cast == turn off immolation")

I'm quite sure that divine shield spell is also quite instant.... I don't remember more with a swift thought...

I agree, you can just use channel and if you change its proprieties decently that will be the only ability you will need to use.
However, for more information about this topic, please read this awesome tutorial, which will surely help you find solutions to this kind of questions:
http://www.wc3campaigns.net/showthread.php?t=81742

Set fade time to -1. Also doesn't interupt channels.

Actually you can by detecting if the unit has the buff on or not of those abilities, you can do it with a periodic check.

ya you can detect by periodic check. what i said is that you cant detect it by just "starts the effect of an ability" for both on/off.