We use SFPs/SFP+ etc. from FS.com and Flexoptix in Peplink products, either coded as generic or Cisco as that is what 99% of our stock is, the switches or Balance 380X/580X I’ve been putting these in have never given me a problem.
Both vendors also offer boxes to recode the optics on the fly, we use both brands in a lot of Juniper, Cisco, Arista, Fortigate etc. without issue after correctly coding them. Both also have warehouses in Germany (and the Flexoptix folks are super friendly and helpful if you really don’t know what to order).
Frankly speaking RJ45 SFPs are the work of the devil, do yourself a favour and either use SX/SR optics if its just to go from one switch to another or use a proper RJ45 port.
If you want 10G on copper then you need a 10G-BaseT SFP+, they exist, they are often also relatively expensive. We pretty much only do 10G or higher on fibre as its frankly cheaper and less hassle.
My reason for stating this is that vendors sometimes implement link detection on the RJ45 SFPs differently, they can totally work just fine (we have lots of Cisco GLC-T in use in Cisco gear) but without testing on Peplink I’d just use some cheap 1G-SX optics and a 1m LC-LC patch cable, the are literally €10 a piece from FS.com
An SFP+ port will typically accept a standard SFP just fine, there are some exceptions to this rule but my experience of Peplink is the SFP+ ports on them are dual rate and will either run at 1G with a regular SFP or 10G if I put an SFP+ in there.
An SFP port will not accept an SFP+.
So for your requirements I’d consider ordering 2x 1G-SX SFP and a short LC-LC OM3 or OM4 patch cable.
https://www.fs.com/products/144158.html (1m OM4 LC/LC duplex cable)
https://www.fs.com/products/75332.html?attribute=947&id=285789 (generic coded SX SFP)
https://www.fs.com/products/75324.html?attribute=945&id=284166 (generic coded RJ45 SFP - not tested these in Peplink myself)
1G SFP SX | 550 m, λ850 nm, DDM, LC-Duplex, Multimode (select MSA standard, or order a Flexbox at the same time for free and do it yourself - they have some caveats about the free box thing so read those first).