In relation to your tracker and losing it if you rent the property, that depends on your mortgage contract. I have a tracker with NIB and it clearly states that I will lose it if I rent the property. Some banks didn't put such conditions in their contracts, some (like mine) did.
In reality, the banks find it hard to determine when a house is rented out, there is no link between the banks and the various govt. departments you have to deal with when you become a landlord. The only way they can tell is from looking at your property insurance cert as it will say whether the house is rented or owner occupied. In my case NIB asked to see this each year for the first 4 years of the mortgage but have not asked for it, so far, this year.
Have a look at your mortgage terms and conditions and see what it says.
In relation to the second point re being a first time buyer, if you buy together and your partner has already bought a house then you lose your first time buyer status.
You could consider buying alone, BUT then the bank will only assess your mortgage application on the basis of your salary and not your joint salary.