First of all, when you run aspnet_regsql it does indeed create a lot of tables by default. However, if you run the tool from a command line, you can specify what features you want to support. Type aspnet_regsql /? for more info.
I don't think it's a good idea to work with SQL Express now if your intention is to switch to Oracle. I think you'll run into a lot of problems with incompatible code when you make the switch.
Writing your own provider is not that hard. And the main benefit of your own provider is that you can easily rewrite the code so it works with Oracle. Take a look here for some suggestions about creating your own provider:
I am not sure, but I wouldn't be surprised if there were already Oracle providers.
If you want to store some additional info besides the (default) user tables, maybe this helps:
With this solution, you can use the standard Login features offered by .NET 2.0 and store additional info in the Profile object.
However, all of this will only work out of the box with a SQL Server database, not with Oracle.
Hope this helps,
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