I'm Reeeeeeaally Not in your climate zone, but here's my understanding as to your question. You certainly can use minis to edge a shrub border. BUT, some minis stay small and others will grow 2, or 3, or even 4 feet. It really depends on the variety. Same thing with winter hardiness. Unless you know the variety you are purchasing, it's hard to know how well they'll survive the winter.

Just from posts I've seen from folks in your zone, you really want to plant roses as early in the spring after frost as possible to give them time to establish a deep root system before they have to deal with a harsh winter. A deep blanket of mulch will certainly be helpful when winter temps come. Others can better advise you on winter protection methods. I never have to deal with that down here in N'awlins. :)

Also, how established is the shrub line already in the ground? If you're planting the minis at the same time as the bigger bushes behind, then they shouldn't have a problem with root competition. But if the shrubs have a lot of roots in the ground already, the minis may have trouble staking thier own claim to root space, unless they've had some time to grow a good root system of their own. Some recommend giving new roses some time to grow in a pot and produce goot roots first without competition from neighboring plant roots before planting them in their permanent spot in the ground. But then you are cutting into some time you'll want for the minis to spread their own roots into the ground to get ready for winter. Maybe if you dig some really good-sized holes for them, cutting out any roots of the shrubs behind in a given space, that will buy them some time to grow their own before they have to compete for root space. June may be too late in the season to want to start your mini border. If you've got some place to do it, you might consider buying potted minis, and overwintering them in some sheltered place such as a garage or cellar before planting them out next spring.
Unless the folks selling the minis you spoke of can tell you the variety, though, I'd advise you to look elsewhere and find out what variety(-ties) you are buying and what their reported zone-hardiness is. If you at least know the name of the roses, you can ask on this forum or check out "helpmefind/roses.com" or local rose societies for the expected bush size and zone hardiness rating. Good luck! I think you idea of edging with minis sounds good. You will just want to plan a little for it so you're minis don't end up as "annuals."

Mike

Thanks so much, Mike. Lots of good info to check out before I haphazardly start putting things in the ground. It is a new shrub border so that is a good thing if I choose to take the chance on planting minis.
I had no idea minis could grow to be not so mini. I guess the "mini" refers to the flower and not the bush?
I saw more minis today on sale for 2.99 for a 6 inch pot but they really looked on their last legs. I think I will do my research first before buying. Thanks for the info on how to find out about hardiness, etc.
Last question, are they prone to fungus or usually more hardy than the Hybrid Tea?
I really, really appreciate your help, Mike. Thanks for taking the time to reply.

Joy

Yes they will blackspot. But with Bayer All -In -One it makes it easy for beginners to grow nice roses. Keep in mind roses are a woody ornamentals! Mini roses have huge root systems, even more so than the bog ones.