If the lavender is fairly good sized and established, it should be fine. I would plant the lavender and let it grow a bit before introducing the mint. also depends on what mint(s) you grow - all are invasive although some varieties are more aggressive than others. You could use lemon balm or anise hyssop instead of the mint. Both self seed although don't spread as aggressively.

If it were me, I would plant an herb garden there and enjoy all sorts of different herbs that attract hummingbirds and butterflies.

On that note, I'd suggest some native wildflowers for the hill and plant your herbs where you can use and enjoy them. For a wild area in PA, I'd probably suggest things like: goldenrod, asters, bee balm, coneflowers, lupines (the wild ones are in bloom here now), compass plant, black-eyed-susan, and more. These all put on a great show for you and the pollinators.

FataMorgana

I live in a very urban area just outside Pittsburgh (actually, my borough is between two Pittsburgh neighborhoods). I already have dedicated herb, wildflower, and vegetable beds, so I am just looking for something that I can plant on the hill and ignore. The reason I want to use lavender and/or spearmint, is that I already have established plants that I can take cuttings from. Budget is a big issue, but my time and labor are free, so I just want to propagate the plants I already have. Plus, the hillside, though relatively small, isn't very accessible for me to go harvest any flowers or herbs on a regular basis.

Mint and lavender would be fine - I would maker sure the lavender is established before introducing the spearmint. You could also plant some butterfly host plants and let the butterfly caterpillars eat them down and use it for a butterfly garden. Seeds wouldn't be too expensive for that

Mint and lavender do not prefer the same conditions. I can't imagine this being an optimal place for both plants.

I suggested a wild flower area since it seemed like a good fit - a steep slope that you wanted to plant and leave alone and get some enjoy out of. If you get the right mix of wildflowers, you will have a nice visual show of blooms all season long. Butterflies too. As a rule you get the most enjoyment out of culinary herbs like these when they are closer to your living spaces and have good access to use or smell them.

By the way, seeds collected and saved throughout the summer and winter-sown is no more expensive than propagating cuttings. It is mostly just your time and labor.

FataMorgana

Depends on your climate. Mountain Mint, an American native grows great in your soil conditions. Lavender will work fine as long as it doesn't get wet. Lavender rarely survives the wet winters here. If you want a low grower, thyme will work.

I'd go for mint. I've never had/seen a lavender act even remotely aggressive. Other "aggressives" to consider in addition to the various mints, that should work in your situation:

Oregano - I've found it MUCH more aggressive than mint.

Creeping Thymes, though not too aggressive, they eventually spread pretty well and keep down weeds.

Daylilies - almost as bad as oregano.

Houseleeks/Hens-N-Chicks. Again, not terribly aggressive, but eventually form very tight mats that weeds cannot penatrate. And there's lots of different kinds to try.

Shasta Daisy. They stink, but they look nice, and will spread very aggressively.

Wormwood - some of these self-seed very aggressively and have very grey, fern-like foliage.

I have 5 rows of lavender on a raised septic mound. The grass is just ruining it. I cannot keep up qwith the week=ding. Is there a ground cover I could use on the paths between the rows of lavender?

I think a groundcover may not be any friendly to your lavender than the grass and you will still have some weeding to do. Perhaps pea gravel over a very breathable landscape fabric may work better for you and be nice looking.

FataMorgana