Rather than replacing the breaker, you should fix the actual problem. This document (PDF) from the Consumer Product Safety Commission, describes a few safe ways to handle aluminum wiring in a home.
Aluminum wiring itself is not dangerous. The danger with aluminum wiring is with how it's spliced/terminated. If connections are not done properly, it can lead to overheating and potentially a fire. If terminations are done properly, aluminum wire can be, and is, used without incident.
Replacement
The most obvious recommendation, is to simply replace all the aluminum wiring with copper. However, that is not always the most practical, or cost effective way to handle the situation.
Pigtailing
If the wire cannot be replaced, the CPSC recommends a few alternative methods to terminating/splicing aluminum wire.
COPALUM Connectors
Using a special connector and tool, joining aluminum wiring to a short piece of copper wire (pigtail) can be a safe solution. This method may require professional assistance, because special tools and knowledge are required.
AlumiConn Connectors
If COPALUM connectors are not available, AlumiConn connectors may also offer a safe connection. AlumiConn connectors are basically insulated terminal block, which allow a copper pigtail to be connected to the aluminum wire. CPSC recommends that this repair also be conducted by a licensed Electrician.
A receptacle without that horizontal slot is only rated to supply 15 amps to a device that's plugged into it. GFCIs are no exception to this. That "20 Amps" stamp is likely stating the receptacle's pass-through rating, or the amount of current it can supply to sockets downstream of it.
There are GFCI devices with that horizontal slot available, which will supply 20 amps to attached devices, and can be found online with a quick Google for "20 amp GFCI receptacle".
You cannot plug a 20 amp device directly into any receptacle without that horizontal bar, be it a regular receptacle, or a GFCI.
Best Answer
Code requires us to follow the MFG instructions per NEC 110.3.B .
Could there be issues using a circuit with multiple receptacles?
Possibly; additional splices or receptacles wired to code might create issues with devices that draw the maximum allowed amount if 1 screw or wirenut is not completely secure. This can create a high resistance connection where it may start arcing. Arcing can damage sensitive electronic components, so even without having anything else plugged in the multiple receptacles could be an issue, even if rare.
Because the instructions state it requires a dedicated circuit, a licensed electrician could not advise you otherwise.