The only soil I had was miracle grow "garden soil" which says "do not use in containers" I looked around a bit and found that its just a heavy soil. I put about an inch of rocks on the bottom and mixed the soil 1:1 with perlite to lighten it a bit.

The munstead is in a 2.5 qt pot right now. I had it in a larger one but read that it prefers a pot that is about 1" larger than the root ball. It came in a basic nursery pot, so I downsized from the large pot I had it in.

Here is a picture included. Do you think the soil will be ok? It's in the sun most of the day. The soil where I live is mostly clay, so I was hesitant to plant it in the ground until it gets larger

First, the rocks in the bottom for drainage is a myth -- it actually impedes drainage and is equivalent to using a smaller pot.

Second, garden soil is made for gardens, not containers. The soil bag doesn't just state this as a joke. It is far too heavy for containers. It is going to retain a ton of water, which again is the enemy of lavender. It is also going to compact and not provide proper aeration for the roots. Garden soil is meant to be mixed in with your existing soil to create a healthy planting medium for in-ground plants.

Third, perlite does not improve drainage -- it retains water. This is not what you want with lavender. You want the most sharply draining soil possible.

I'd hate to be the bearer of bad news, but just about everything you did is going to make your lavender more likely to perish. You want to get a very well draining gravelly soil such as cactus/succulent soil that preferably is a bit alkaline.

Lavender will do just fine in clay unless it is poorly drained (meaning water doesn't leave a 1ftX1ft hole in over an hour). Clay soil is not necessarily poorly drained. I can understand about wanting to get it established before planting in the ground, but it should be fine if you use some soil conditioner and gravel. If your soil is poorly drained, getting it established in a pot isn't going to make it more likely to survive poor drainage. They can't deal with waterlogged soil, especially in winter.

Usually I do a bit of research before hand, but it was a spur of the moment buy, and I just went off what the nursery lady told me. I'm so glad I asked instead of taking her word at it.

I have one planted in what is basically a spoil heap of silt & rubble.

The garden soil/perlite mix actually dried out pretty well and wasn't soggy, but ill swap it out any ways and use the garden soil for the irises I want to put in the ground.

It is vermiculite one needs to be cautious with, especially in container soils. It does absorb water (to more than 300% of its weight) and once fully saturated, collapses, compromising both drainage and aeration.