Regarding the question about Tongyong for Taiwanese, the answer is yes, kind of.
As Chris says, any romanisation system can be adapted for different languages. There is an adaptation of Tongyong for Taiwanese, but likewise there’s an adaptation of Hanyu Pinyin for Southern Min (it’s called Pumindian and it was produced by the University of Xiamen). This is not a “strength” of Tongyong.
In fact, I’ve written about this on my site, so I’ll just quote that here:
[quote=“Taffy, on Tailingua”]POJ: Thōng-iōng Peng-im
Chinese: 通用拼音
The inventor of Tong-iong, Yu Bo-quan, billed it as a universal romanization for all of Taiwan’s languages (the name Tong-iong actually means ‘use for everything’). In actual fact it is modified to fit Mandarin, Taiwanese, Hakka and so on, so it can’t really be said to be a single system, rather it’s a collection of linked systems for different languages.
As a positive point for English learners, Tong-iong maps more closely to English orthography than the POJ system - a ‘d’ in Tong-iong is pronounced similarly to an initial ‘d’ in English (POJ would have ‘t’ in the same place).
One controversial aspect of Tong-iong for Taiwanese however is that it represents tones after the sandhi changes, whereas every other system detailed here describes the tones in their original state and leaves the reader to apply the sandhi and when required. This makes Tong-iong easy to read out, but is a disadvantage for learners in that same syllable will be written differently in different positions in a sentence.
See a comparison of the major romanization systems[/quote]
I was probably a bit too diplomatic there: I think that the sandhi issue really hamstrings Tongyong for Taiwanese.
In order to adapt to Taiwanese, Tongyong needs to add a number of new letter or combinations of letter: nasal vowels (ⁿ), stops (h, k, p, t on the end of syllables), the intials bh and gh, m as a final, the vowel “or” and more besides. It also re-uses combinations from Tongyong for Mandarin but gives them different pronunciations, for example “e” is pronounced something like “uh” in Mandarin, but then for Taiwanese you have to read it as the Mandarin “ei”.
So it can’t really be said to be the same system.
Yup. Even the Ministry of Education under the DPP rejected Tongyong and plumped for Tai-lo instead.