Can I make my podcast (that I made) appear oldest to newest

You should be able to do this with the 'itunes:type' tag. Apple say:

'serial. Specify serial when episodes are intended to be consumed in sequential order. Apple Podcasts will present the oldest episodes first and display the episode numbers (required) of each episode.'

This tag goes in the top section of the feed, (before the first 'item' tag) and would look like this:

<itunes:type>serial</itunes:type>

And within each episode's tags you would have:

<itunes:episode>4</itunes:episode>

showing the relevant number.

I don't have experience of this so I don't know if it works identically both for subscribers and for people using the Store page, but one would think it likely.

I'm not familiar with SoundCloud but I would assume that they construct the feed from the episode data and recording you provide and you don't have direct access to modify the coding. These tags are fairly new and it seems likely that Soundcloud won't provide a way of entering them, though you could try taking it up with them. The problem is that most podcasting services don't let you modify the feed directly (with some reason since even a minor error can wreck it). Given this I doubt there is anything you can do. FWIW most podcast users are used to the 'latest on top' display. Numbering your episodes clearly in the titles is essential, though.