To stay true to form with a MD you would not use rail buttons or any type of rail guide, although if your not going for any records I would say do whatever you feel comfortable with. The low profile universal rail guide is a good choice. I have several MD rockets, including the original Mongoose 38 and none have rail buttons. I have used either a club tower or the fly-away-rail guides in both the 38 and 54mm config. I know some have reported issues with the rail guides, which should not be dismissed so quickly, however I have not had an issue and still have full confidence in the technology (currently looking for some wood to knock).
Other considerations you will have with this build is motor retention, which you are going to get a whole wack of opinions on. These will vary from friction-fit to traditional retainers and internal MD retainers like an Aeropack (which is what I use), and then there are the custom homemade retainers. If you have access to a 3D printer you might want to look around the forum. I was looking at a 38mm MD GoDevil build that the OP did up a really nice simple 3D printed internal retainer. Plus and minuses of these vary. Friction-fit is the KISS method so simple low-to no cost and a lot of guys swear by it, I have never been comfortable but I wouldn't dismiss it so fast. Traditional retainers are relatively low cost and allow any size hardware to fit, without the need to use extension rods or something that is removable and relocatable. However traditional retainers have a bit of external bulk that goes against the premise of MD. Again this is or isn't an issue depending on the flyer. Internal MD retainer is reliable and holds true for MD, but it is the least flexible without a bit of thought, which equals more work and it may require the addition of extension rods so you're caring a bit of extra mass with smaller or shorter hardware.
Another relative challenge for a 38mm MD is electronics. You will want to go with electronics vs. motor eject, IMO, and you will want a tracker. Packing that into a 38mm air-frame is very doable but it will require a bit of extra thought vs. say a 54mm or larger air-frame. Again lots of threads will show you some really great ideas, and today there are some good 3D printed solutions for both. I used a 3D printed sled for the altimeter section on my Mongoose 38, and went with an EggFinder mini in the nosecone for tracking. Again lots of discussion around this setup. Even to the point where if you were going for max performance you may consider more of non-traditional single split HED config.
One of the more controversial discussions will be fin attachment, specifically T2T or not. Again this is personal and I would do first what is aligned with your goals and or just what you want to do. With a 38mm there certainly is enough flyers who have successfully launched with the bare min, I am not one of them. I have done T2T on all my MD builds, mainly becasue it is aligned with long term goals of launching something that will really requires it, and it does not make any sense to me that the first time I am using a technique is on a build where it will really count. Therefore part of why I have done this is to gain practical hands on as well as to refine my technique a bit, any with T2T you will change up your method with each layup, certainly for the first x number of attempts.
At the end of the day there are a lot of considerations with MD vs. a traditional rocket and not just in fin mounting, but first and foremost your decisions are yours so have some fun with it.