If you have a mix which works for your other plants that will be fine for your Begonias. Humidity and warmth are more important than any special soil. B maculata is itself a 'common houseplant'. There's nothing particularly special about it and I'd consider it much easier than the Philodendron.

I bought the same plants this year and they are from Costa Farms which use only coir in these pots. I let my maculata alone (for now), but when I repot it will go in my go to mix - MiracleGro Potting Mix. I discovered that for winter growing under lights that I have much better results by watering lightly instead of a deep watering.

On to the philodendrons, I have started using a mix of orchid potting mix (I found Lowe's brand that they sell better than Home Depot's orchid mix). I use about 2/3 orchid mix and 1/3 MiracleGro potting mix. Check out Sydney Plant Guy on YouTube - he gives excellent advice.

Also, Costa Farms has grown some plants in the past in those plastic mesh plugs. They claim they abandoned that practice. I don't mind them starting plants in this mesh, but when potting up for sale, DEFINITELY remove the mesh. I potted up over 20 philodendrons and other plants this year from the big box stores and Etsy - some have mesh and others don't.

Here are some photos of my maculata and Painted Lady. These are sitting on my covered front porch that receive some direct sun and then bright light from sunrise to sunset.

Painted Lady in the basement under LED shop lights. This is before I repotted. The looks has not changed since the repot last week. As long as it didn't wilt, I am happy.

Here are four pots that I potted up to the chunky mix weeks ago. I use clear pots to see roots developing as well (thanks to the Sydney Plant Guy SPG)

This is the same mix I used for this ornamental yam (Dioscorea discolor)

and Anthuriums

I am trying to replace all the original plants with clear pots and chunky mix.

This P. tortum is the only plant I took photos of last week when I repotted 12 aroids. Note the wick that Costa Farms uses these days.

I've had this plant for over a year (bought at a Walmart in TN)

Look at the length of that main root!

Gotta Go - the wife wants to go shopping now.

My Begonias did not grow well in coir 'peat'(it's called fine coir dust too). part coir 'peat' part peaty indoor mix is ok with lots of perlite added (at least one third of mix, perhaps more): begonias have such fine roots that i don't add fine-bark/coco-chunks.

My maculata has grown despite growing in coir. Eventually I will replace it with my go-to Miracle-Gro potting mix. At least, growing it under the roof of the front porch, I can control how much water it gets.

This is how it looked two days ago (facing west)

facing east

I think B. maculata is on steroids!

what sort of B. rex is the one in front? it's gorgeous!

I don't have the name of the rex. It is one I got at a Lowe's Store this year in TN.

Here are some of the pix I took back in June while shopping there.

The bigger pots cost a lot more. Notice the one on the right is the one I bought for around $13 or so. The big ones were $25 or so.

I bought a gallon sized maculata from PHOE in April 2009 when the begonia convention was hosted by West Palm Beach. Here it is in an 18" pot sharing space with other canes. I grew several canes from seed collected from the mother plant. I lost the mother plant (back when I didn't know that much about mildew and its effect on polka dot canes and other mildew sensitive begonias)

How it looked when I bought it.

Some at PHOE that I had difficulty choosing one.