I just bought a Tomislav Killstreak Kit (Not professional or specialized; I use directx 8.1) for my strange Tomislav just to give it a bit more appeal. However if I use it on my strange Tomislav, will it become a Strange Killstreak Tomislav or just a Killstreak Tomislav?
You can make weapons be 'Strange' and 'Killstreak' weapons at the same time, using the Killstreak Kit will not remove the Strange quality.
If you apply the Killstreak Kit to your Tomislav it would be named <Strange> Killstreak Tomislav, with 'Strange' being whatever your current Strange rank is.
From the Killstreak Kit Wiki page:
Killstreak Kits can be applied to weapons of any quality. If a Killstreak Kit is applied to a stock item, it becomes a Unique item and the player will subsequently find it as an item drop.
According to the answer on How many name prefixes can be stacked onto one weapon?, the only weapon qualities that cannot interact at all are:
This means you cannot have a 'Festive Australium' weapon, but you can have a 'Strange Killstreak' weapon, or even a 'Strange Festive Killstreak' weapon.