Using the SSi method, is using a flash card system (like Anki) for the words and phrases introduced advisable? I saw that in the old system, a list of Vocabulary was provided for each challenge, but do not see anything similar in the app.
For two follow-up questions:
1.) When should one (if ever) begin to incorporate grammar instruction and reading into their learning?
2.) Is it considered a bad idea to learn more than one language at a time, using this method?
1
In general, if you incorporate reading too early, you tend to want to use your native language phonemes for the letters. Keeping it aural for as long as you can, you start to hear the target language phonemes.
The flip side of this is sometimes you don’t hear the correct one. In the beginning, I was always hearing ‘V’ instead of ‘DD’. So, occasionally looking up the spelling for a word can keep you on track, just try not too over use it.
2
The brain (especially young ones) has a remarkable ability to differentiate languages. It will take a bit longer to reach fluency, but give it a whirl if you have the gumption.
I have been an adult learner for decades and a ‘natural engineer’ at heart, constantly wanting to understand ‘why’ something’s like it is. I’ve completed SSIW (north) and reached mid-point in south black belt (for revision and awareness of what might come up on Welsh visits). I’ve also completed Duolingo.
I see no issues with reading grammar and using Anki (you could use the vocabulary lists from the old course) - it’s like looking at something in 3D 
(the Duolingo notes or the BBC Welsh grammar book are good starting points).
I also support doing more than one if you feel inclined - I find there’s a slight delay in my head when switching (I generally focus on Welsh but do either Italian or French in the shower) as if the brain is literally using a slightly different set if neurons!
I do sometimes wonder if I’d be able to say the phrases if Arun’s voice wasn’t there prompting me though!
Go for it!
This is why it’s important to start creating sentences for yourself as soon as you can. When you’re not actively using the app, try to think up short sentences, progressing to describing what you’re doing, or what you did yesterday etc - gradually making the language your own and not relying on prompts.